Defrost the Beef Wellington in the refrigerator overnight.
Position a rack in the upper third of the oven, and preheat to 400F. Place the Beef Wellington on a greased baking sheet. Bake until the pastry is golden brown and a meat thermometer inserted into the center registers 130-135F for medium-rare (22 minutes) or 145F for medium (25 minutes). Let the Beef Wellington cool for five minutes before serving. Appliances vary, so be sure to adjust the time and temperature accordingly.